This invention relates generally to color television and stereophonic broadcast receivers and more particularly to the preference controls thereof. Color television receivers provide a number of viewer acccessible preference controls, among them being controls for the adjustment of receiver characteristics such as contrast, brightness, hue, color saturation, volume and in some cases, video peaking or sharpness. Similarly stereophonic receivers provide preference controls for volume, balance, bass and treble. The difficulty of properly setting such a variety of controls for the average unsophisticated user has stimulated development of automatic systems for the correction of transmission caused errors and preset control systems to assist the viewer in adjusting receiver preference controls.
Automatic systems generally comprise receiver circuitry which is responsive to changes or aberrations of the received signal and affects appropriate compensation to minimize the degradation of receiver performance caused by such aberrations. Such systems are not generally accessible to the viewer and do little to compensate for misadjustment of receiver preference controls but rather are directed toward maintaining proper reception and recovery of the broadcast information. Stereophonic receivers often provide such automatic systems for maintenance of a substantially constant audio level and station tuning. Color television receivers may provide automatic systems operative on the receiver characteristics of color saturation, hue, brightness, contrast and channel tuning.
Because these automatic systems do little to compensate for misadjustment of receiver preference controls, television receiver manufacturers often provide a control system which enables the viewer or operator to impose a predetermined set of characteristics upon the receiver or at least to confine the controls to within a desirable operating range. These take the form of a non-interactive set of duplicate controls (not accessible to the viewer) which are preadjusted by the manufacturer, and circuitry which limits the available range of operation of the viewer accessible preference controls around a predetermined adjustment. One manufacturer intercouples the duplicate controls to provide a limited range of adjustment of the receiver characteristic in the preset mode.
While the use of such systems provides some assistance to the viewer who has misadjusted the receiver preference controls, the imposition of predetermined characteristics does not change the preference control adjustment and upon a return to preference control operation, the previous misadjusted condition returns. Further, in the preset arrangements incorporating non-interactive duplicate controls, there is no provision for modifying the imposed characteristic. In the event that the presets no longer represent desirable characteristics due to component aging, environmental conditions, program material, or in the event the viewer desires a "set up" which differs from the "norm" of the presets, there is little short of a service adjustment that will be of help. The interactive systems and limited adjustment systems do provide for a measure of change in the receiver characteristic imposed, but the changes are generally insufficient to accommodate the existing variations of viewer taste. Moreover, because the effect of the control and not its range of movement is limited in the preset mode, touch up adjustments often result in even greater misadjustment of the control when returned to preference operation.
The preference controls most often used are potentiometers of either the rotary or slide type which include a contact movable over a resistance element. Such devices are susceptible to mechanical breakage and in time may become noisy or intermittent. Additionally they are not easily nor economically adaptable to remote control applications, because expensive servo-motors are needed to drive the movable contact.
Electronic control systems obviate these mechanical problems and are finding increased use in the television industry. They are silent, reliable, compact and readily adopted to remote operation. The development of electronic control systems has been facilitated by recent improvements in receiver preference control circuitry that enables changing a given receiver characteristic in response to a DC control voltage. Prior art preference controls were often operative on the processed signal itself and comprised portions of the signal path connected to the receiver chassis by expensive and unwieldy shielded cables. The advent of DC responsive systems permitted the elimination of such signal bearing connections and made practical preference control systems which are electronic rather than mechanical.
In one of the presently used electronic systems, a memory capacitor coupled to the input electrode of an amplifier is alternatively charged or discharged by operator preference controls and provides bias for the amplifier. A field effect transistor amplifier provides a DC control voltage which is applied to a voltage controllable circuit element in the receiver. The viewer has access to a pair of push button preference controls which enable adjustment of the receiver characteristic in opposite directions. For example, the color saturation control comprises a button for increased color saturation and a button for decreased color saturation. Viewer adjustment consists of pressing the appropriate button until the color saturation reaches the desired level. Once the desired adjustment is made, the system remains constant until further adjusted because the capacitor coupled to the input (gate) electrode of the FET amplifier remains charged due to the inherent low leakage characteristics of the FET.
Though presently more expensive, recently developed digital electronic control systems exhibit advantages over capacitive memory systems in the areas of reproducibility and ease of automated production. A digital approach to electronic control systems comprises the use of a local clock pulse generator for providing a train of clock pulses, applied via a preference control, to a reversible binary counter. The viewer preference control determines the duration of clock pulse application, that is, the number of pulses applied, and whether pulses cause the counter to "count up" or "count down". The output signal of the counter is a binary representation of the arithmetic sum of clock pulses which have been applied. For example, if 10 clock pulses are coupled to the counter causing it to count up, the output of the counter would have a numeric equivalent of 10 expressed in binary code. If 6 clock pulses are then coupled to the counter causing it to count down, the counter output would have a numeric equivalent of 4. The output of the counter is coupled to a digital-to-analog converter which produces a DC voltage suitable for application to a voltage dependent circuit element. When clock pulses are applied to the counter, a binary coded output corresponding to the number of pulses applied results. The output voltage of the digital-to-analog converter (the control voltage) increases in proportion to the number of clock pulses applied.
Operation and appearance of the digital system are essentially the same as discussed above for the capacitive memory system. The direction in which the control voltage, and hence the receiver characteristic, is changed is controlled by viewer selection of either an "up" or "down" button and the degree of change is controlled by the length of time which that button is depressed. Because the clock pulse generator can be controlled by electronic switching signals, such digital systems are readily adaptable to remote control operation. In the absence of clock pulses, the counter output signal, and the control voltage will remain unchanged. As a result, once adjusted to viewer preference, the system maintains its adjustment.
While such digital control systems offer advantages, there are some difficulties encountered, among them, the random starting point of the controls after periods of receiver deactivation caused by the loss of binary information stored in the counter. Current solutions to this problem require continued power application to the control system during periods of receiver deactivation.